ATP (adenosine triphosphate) is the universal currency of energy. It is a small molecule with 3 phosphate groups (P) attached to an adenosine molecule i.e. Adenosine-P-P-P

During respiration, high energy C-C, C-H and C-OH bonds are broken. Lower energy bonds are formed and the difference is released and used to attach a P to Adenosine-P-P (ADP adenosine diphosphate), making ATP.

When energy is required at a later time by a cell, it can use the ATP and break a P off the end. This releases the energy needed (30.6kJ for every ATP, ADP + P). The more ATPs used, the more energy is released.

For aerobic respiration to occur, the cell needs to possess mitochondria.

Many of the reactions in the stages of respiration involve oxidation by the removal of electrons or hydrogen atoms (H). These are transferred to electron/hydrogen carriers. Ultimately they are passed to oxygen to form water right at the very last stage of respiration.